Sleeping warm: sleeping bag temperature ratings and sleeping pad R-values

A cold night in a tent is rarely just a sleeping bag problem. The bag insulates you from the air above; the pad insulates you from the ground below. You need both. This guide explains how to read the two numbers that matter, the ISO temperature ratings on a sleeping bag and the ASTM R-value on a pad, how altitude changes the forecast, and how the same setup carries over to a power outage or an evacuation shelter.

Sleeping bag temperature ratings (ISO 23537-1)

Many sleeping bags carry temperature ratings tested to the international standard ISO 23537-1. The current edition, published in March 2022, covers adult sized bags "for use in sports and leisure time activities" with a limit temperature of −20 °C (−4 °F) or warmer, tested in steady-state conditions. ISO says it does not apply to bags for "military use and extreme climate zone expedition," or to bags for children or babies.

The standard's draft (ISO/DIS 23537-1, 2015) defines the three numbers you usually see like this:

So the extreme rating is a warning line, not a range you can count on. Our suggestion: pick a bag whose comfort rating is at or below the coldest night you expect, and treat the limit rating as a "cold but okay" margin. (We could not check the wording of the final 2016 or 2022 text, which is behind a paywall.)

Sleeping pad R-value (ASTM F3340)

R-value is the pad's thermal resistance: higher means more insulation from the ground. ASTM F3340, adopted in 2018 and now in its 2022 revision, measures it by holding the pad "under constant compressive force between a guarded hot plate and a cold plate," so heat flows "vertically through the camping mattress thickness." ASTM says the result is meant to help consumers compare and select pads. NEMO, a U.S. pad maker that helped write the standard, says the test standard was "ratified in Fall 2019" and that it then started publishing R-values for all its pads. Older pads, from any brand, may not list a comparable number (our note).

Two caveats from the standard itself and one rule of thumb:

• ASTM calls the number "a repeatable comparative measurement" that "will not always correlate to actual insulation performance," because of things like "edge heat loss, uncovered surface heat loss, compression rate changes or posture changes during sleep."
• The test measures one pad alone: "Auxiliary insulation of any type shall be excluded in the measurement."
• NEMO points to "the ability to calculate the precise, additive R-Value of multiple pads stacked in use (such as an air pad on a closed-cell foam pad)." Stacking a foam pad under an air pad is the classic way to get a warmer bed for cold trips, and the foam is a backup if the air pad leaks.

Why the ground matters

NOLS, the outdoor leadership school, puts it plainly: "Utilizing a sleeping bag or pad adds crucial insulation from the cold ground, which can rapidly draw heat away from the body." A small 2025 rescue study (seven volunteers on a spineboard placed on a concrete floor in a −10 °C chamber) found that "heat conduction towards the ground accounts for approximately one-fifth of the total heat loss." That setup is not a campsite, but the direction is clear: on a cold night, adding a pad often helps more than a thicker bag (our view).

Altitude: the forecast is for the valley

A National Weather Service training document puts the average cooling with height at "2 °C (3.5 °F) per 1,000 feet," and the FAA's aviation weather handbook gives the standard atmosphere as "6.5 °C/kilometer (3.57 °F/1,000 feet)," adding that "since this is an average, the exact value seldom exists." Japan's weather agency gives a range of 0.5 to 1 °C per 100 m. In practice, a campsite 3,000 feet above the town in the forecast can easily be 10 °F colder (our arithmetic). Check the elevation of where you'll sleep, then choose your bag.

After a disaster: the same bed on a shelter floor

Japan's shelter guidelines now push to get evacuees off the floor. The Cabinet Office guideline for evacuation shelters (revised December 2024) says shelters should stock partitions and simple beds, including cardboard beds and air beds, and set them up when the shelter opens, because lying on the floor for a long time can cause "economy class syndrome" (deep vein thrombosis) and health problems from breathing in dust. It also sets a minimum of 3.5 m² of living space per person, citing the Sphere humanitarian standards.

Japan's health ministry, in guidance issued after the 2011 earthquake, noted that sitting on a mat or tatami instead of directly on the floor helps against the cold, and that once someone starts shivering they should get insulation under them, get out of the wind, take off wet clothes and wrap up in blankets. Tokyo's stockpiling guide lists a sleeping bag and mat among the camping gear that is useful in a shelter. Your camping sleep system is your power outage and shelter bed, too.

For hypothermia warning signs and first aid, see hypothermia and frostbite. Never try to heat a tent or car with a stove or heater: see carbon monoxide at camp.

Hot water bottles and hand warmers: low-temperature burns

Japan's Consumer Affairs Agency warns about "low-temperature burns" from hot water bottles, heat packs and electric blankets used in bed. Its guide to skin damage: about 3 to 4 hours at 44 °C (111 °F), 30 minutes to an hour at 46 °C (115 °F), and 2 to 3 minutes at 50 °C (122 °F). Its advice: warm the bedding with a hot water bottle, then take it out before you sleep; don't sleep with disposable heat packs on; don't run electric blankets on high; and see a doctor for any pain or unusual feeling, since these burns can be worse than they look.

Our suggestions for a warmer night

Our Japanese edition has a separate article on the same subject, written for Japan: 寒くない寝床のつくり方.